This invention relates to a liquid coating composition for forming a hard and abrasion resistant coating film on a plastic member and a method of forming such a coating film on a plastic member which is to be shaped after the application of the liquid coating composition thereto. The liquid coating composition is of the type using an organotrialkoxysilane and colloidal silica.
It is prevailing to use a synthetic resin high in transparency, such as polymethyl methacrylate or polycarbonate as a so-called organic glass. For plastic articles using a plate of such a resin it is almost indispensable to protect the plate surface with a coating film which is transparent, hard and resistant to abrasion and scratching. For example, it is well known to form a satisfactorily hard and transparent coating film by using a liquid coating composition containing an alkoxysilane and colloidal silica as the principal components.
When a plastic plate is to be shaped and provided with a hard coating film, the coating is made before and after shaping the plate. In the case of first forming the coating film on a flat plate the coating operation is easy, but in this case it is necessary to afford some flexibility and pliability to the coating film so that the coating film may not be damaged by shaping the plastic plate under heating, and therefore the surface hardness of the coating film is inevitably sacrificed. In the case of forming the coating film after shaping the plastic plate it is possible to form a sufficiently hard coating film, but in this case it is often that strict restrictions are placed on the coating method by reason of intricately curved shapes of the surfaces to be coated, and it is very difficult to form a coating film uniform in thickness and smoothness.
To bend a plastic plate having a hard coating film, there is a proposal of forming a linear notch in a coated surface of the plate to a depth greater than the coating film thickness, bending the plate along the notch such that the notched surface becomes the outside surface of the bent plate and thereafter machining the notched region of the outside surface to eliminate the notch by reducing the thickness of the plate in that region. However, this method entails complicated operations, and by this method the notched and subsequently machined region becomes relatively low in transparency, and it is impossible to form a gently curved surface.
JP No. 63-30934 proposes to accomplish shaping and hard coating of a polycarbonate plate by first applying a liquid coating composition comprising methyl-etherified methylolmelamine, glycol, hydrolytic partial condensate of an alkoxysilane and a curing agent to the flat plate, partially curing the coating film by heating at 120.degree.-150.degree. C. and then bending the plate at 150.degree.-180.degree. C. to fully cure the coating film during the bending operation. However, the surface hardness of the obtained coating film is insufficient since a melamine resin is used as a principal component of the coating material.